Responses

It's eyecup FG. Common to most of the ZX cameras, except for the ZX-M which is too cheap to have an eyecup.
Easiest way is to probably buy one the cheap "double digit" ZX cameras with the "crippled" lens mount that still has it. Or one with a shot mirror motor gear.
My own MZ-3 came without one, haven't addressed the problem yet. I did buy a ZX-M for the split-image screen and the non-Quartz-Date back.

For what it's worth, most K-mount Pentax cameras had the same viewfinder accessory mount, though the eyecup style changed a bit over time. Search e-bay for "pentax eyecup", you'll see a lot of choices.

Eyecup Fk was on some of the other MZ/ZX cameras like MZ-6/ZX-L - you probalby don't want this one, it was sort of small and hard; the Fg is more like the Fo for the K10D with a softer rubber surround. The FL (for digital *ist series) looked a lot like the Fg too, a little smaller than the Fo.